1. Field of the Invention
The present invention relates to an ion-conductive polymer electrolyte and a method for producing the same, and further relates to an electrolytic capacitor as well as an electric double-layer capacitor configured with the same electrolyte for driving the capacitors.
2. Description of the Prior Art
Conventionally, as an electrolyte for driving an electrolytic capacitor, a solution prepared by dissolving an ammonium salt as a main solute in an organic solvent such as ethylene glycol has been used. However, there has been a possibility of leakage and escape of the electrolyte by evaporation in the capacitor using such liquid electrolyte, and hence it has been difficult to obtain a long-term reliability in operation in such capacitor.
In order to solve these problems, there has been proposed an electrolytic capacitor free from the leakage and escape of the electrolyte by evaporation, configured by solidifying the capacitor element with the use of an ion-conductive polymer electrolyte composed of a mixture of siloxane-alkylene oxide copolymer and polyethylene oxide, as its mother ingredient, and an alkali metal salt, instead of the liquid electrolyte.
However, the electrolytic capacitor using the ion-conductive polymer electrolyte with alkali metal ions as its mobile ions has a disadvantage that the alkali metal ions are liable to diffuse into a dielectric substance layer formed on an anode of the electrolytic capacitor, and the thus diffused alkali metal ions might sometimes cause a lowering of the dielectric constant of the dielectric substance layer, and finally invites a short-circuiting in the capacitor.
In order to overcome such disadvantages and deficiencies, it has been considered to use ammonium ions in place of the alkali metal ions which had been used as the mobile ions of the electrolyte for the electrolytic capacitor. However, it has hitherto been known that the ion-conductive polymer electrolyte which contains ammonium ions has a very low ionic conductivity in general.
All of the various polymer electrolytes which have hitherto been proposed have a disadvantage that their ionic conductivity seriously decreases when their operating temperature is lowered from room temperature to 0.degree. C. or lower.
When such an electrolyte having a low ionic conductivity is used for a capacitor, the impedance of the capacitor element becomes large. And, the application of such capacitor is drastically limited because of its power loss and heat generated during the operation. Thus, the electrolyte is difficult to use from the practical point of view.
In order to enable the use of such an ion-conductive polymer electrolyte for driving an electrolytic capacitor, it is essential to clarify a suitable combination of various polymer mother ingredients with electrolyte salts for realizing an electrolyte of high ionic conductivity, but no concrete or specific example has not been clarified so far.
In addition, the application of the aluminum electrolytic capacitor has recently been expanded widely, and its long-term stability or reliability during high-temperature storing has been attracting attention in this art. For instance, a guaranteed quality for the continuous exposure for 10,000 hours at 105.degree. C. is required by the current market. When the polymer electrolytes are exposed to such high temperature atmosphere, a physical and/or chemical deterioration such as crack, contraction or dissolution may be produced, and thus the exposure may cause a serious deterioration in the characteristics of the capacitor element. A solid electrolyte which does not suffer from any deterioration in its performance under such severe test environments has not been proposed yet.
Recently, an electric double-layer capacitor configured with an electrolyte comprising sulfuric acid or an organic electrolyte has been employed for various electric or electronic appliances. An electric double-layer capacitor configured with an electrolyte comprising sulfuric acid has a disadvantage that its decomposition voltage is as low as about 1.2 V, which is lower than an electrolysis voltage of water, but the conductivity of the electrolyte is as high as 0.7 S/cm. By utilizing this characteristic, the electric double-layer capacitors are employed for such applications wherein a relatively large output current is required, for instance, for emergency power source in case of a cut-off of usual power source (see, for instance, NEC's Technical Report, Vol.44, No.10, 1991).
On the other hand, an electric double-layer capacitor configured with an electrolyte comprising an organic electrolyte composed of, for instance, propylene carbonate as the solvent and tetraethylammonium perchlorate as the solute has a decomposition voltage of 2.4 V, which is twice as high as that of the capacitor with sulfuric acid. The ionic conductivity of the electrolyte is 0.01 S/cm, which is lower by about two orders as compared with that of the sulfuric acid. By utilizing this characteristic, the electric double-layer capacitor configured with an organic electrolyte has been employed as, for instance, a back-up power source for semiconductor memories in miniature electronic appliances (Carbon, No.132, pp.57, 1988).
The capacitor configured with such liquid electrolyte is however liable to cause a leakage of its electrolyte, and thus a capacitor configured with a solid polymer electrolyte has been proposed as the one essentially free from the leakage.
The configuration of such capacitor is exemplified as one wherein an electrode is produced by impregnating a porous carbon material with an electrolyte of polyvinyl alcohol solution containing a lithium salt such as lithium perchlorate (J. Power Sources, 36, pp.87, 1991), or one wherein an electrode is produced by mixing an electrolyte of polyethylene oxide containing an alkali metal salt such as lithium perchlorate with activated carbon (Japanese Laid-Open Patent Publication No. Hei 2-39513).
As described previously, there has currently been proposed a solid electrolyte configured with a base polymer containing a lithium salt such as lithium perchlorate, as a typical configuration of the solid polymer electrolyte having a relatively high conductivity. However, if an electrolyte is configured with an alkali metal salt such as lithium perchlorate for assembling a capacitor element, it is indispensable to completely remove water from the rest of the components constituting the capacitor element. It is very difficult to completely remove water from the porous carbon such as activated carbon which is a structural component of the electrode and to assemble the capacitor element in the completely water free state. From the practical point of view, the difficulty in the assembling process has also hindered a realization of such a capacitor so far.
At present, a sealing operation of a thin-type capacitor element has been performed using a compound material composed of a metal foil such as aluminum foil having an inner face laminated with a layer of electrically insulating sheet made of, for instance polypropylene, in general. However, the present inventors have confirmed that if a sealing of such a structure is performed on a capacitor element configured with the above-mentioned lithium salt electrolyte and the sealed capacitor element is stood still under such an atmosphere at a temperature of 60.degree. C. and a relative humidity of 90%, deterioration in the performance of the capacitor element appears after about two months. The cause for the deterioration is considered to be water which gradually invades the capacitor element through the sealing material.
As a means for solving these problems, there has been considered an employment of an ammonium salt, which is the same as that used in the above-mentioned organic electrolyte, as the salt for configuring the solid polymer electrolyte. The ammonium salts in general are however difficult to be dissolved in the polymer compound such as the above-mentioned polyvinyl alcohol and polyethylene oxide in a large quantity. As a result, the ionic conductivity of the polymer electrolyte configured with an ammonium salt and polyethylene oxide is very low (Naoya Ogata: "DODENSEI KOHBUNSHI (Electrically-Conductive Polymer)", KOHDANSHA Scientific, 1990).
The ionic conductivity of the electrolyte constituting the electric double-layer capacitor acts as a resistance of the capacitor, and when the ionic conductivity of the electrolyte is too small, only weak output current is obtained, and the use of such electrolyte in the electric double-layer capacitor is difficult from the practical point of view.
More important fact is that the capacitance of the electric double-layer capacitor is directly proportional to the concentration of ions in the electrolyte, and there is a problem that the capacitance of the electric double-layer capacitor can not be made large if it is configured with the electrolyte containing such a small amount of ammonium salt.
In addition, the electrolyte salt employed in the electric double-layer capacitor is required to creep into micropores of about several tens of angstrom in size existing in the porous carbon, which is a structural component of the electrode, and thus the electrolyte should be in such a molecule that produces ions having a radius as small as possible. However, it is well known that the smaller the size of the anion and cation of the salt is, the harder to dissolve the salt in the base prepolymer (KISODENKIKAGAKU SOKUTEIHOU (Fundamental Measurement in Electrochemistry) issued from Electrochemical Society of Japan, pp.30, 1981).
Thus, the use of a polymer electrolyte containing a lithium salt in the electric double-layer capacitor poses a problem of complicated water removal in the manufacturing process and necessarily requires a water-tight sealing material under an atmosphere of high temperature and high humidity.
When a polymer electrolyte containing an ammonium salt is employed as the electrolyte for the capacitor as a means for solving such problems, it is very important to realize an electrolyte having a high ionic conductivity, by dissolving a large quantity of salt having a smaller cation/anion radius in the polymer, and by selecting a suitable combination of the polymer mother ingredient with the ammonium salt; however, no concrete or specific configuration which fulfills all of the above-mentioned requirements has not been proposed so far.